The invention relates to a particular quality of metallurgical silicon with a controlled structure and containing aluminum, intended particularly for synthesis of alkyl or aryl-halogenosilanes used in the manufacture of silicones.
Metallurgical silicon is silicon obtained industrially by carbothermal reduction of silica in an electric furnace. It contains at least 98% silicon, and the other main elements are iron, aluminum and calcium. It also contains some oxygen and other elements such as P, Ti, V, Ni, etc., with a content of  less than 0.1%.
The synthesis of alkyl or aryl halogenosilanes by the reaction of a halogenated hydrocarbon on silicon at a temperature of between 250 and 350xc2x0 C. in the presence of a catalyst has been known since U.S. Pat. No. 2,380,995 issued to Rochow in 1945.
Rochow""s reaction has been developed industrially to a large extent because it is the basis of the silicones industry. It is usually used with methyl chloride CH3Cl and gives a mix of different methyl chlorosilanes, particularly monomethyl-trichlorosilane (denoted by the letter T) and dimethyl-dichlorosilane (denoted by D). Since the required product is D, it is important to carry out the reaction so that a maximum proportion of D is obtained in the resulting mix of silanes, this proportion being called the selectivity of the reaction. It is also important to produce the maximum quantity of silanes per unit time, the value of the weight of silanes produced per unit time being called the reactivity.
A large amount of work has been carried out to improve the reactivity and selectivity of the reaction. In particular, the role played by intermetallic compounds present in the structure of the metallurgical silicon used as a raw material has been emphasized. For example the publication by the applicant T. MARGARIA, J. C. ANGLEZIO and C. SERVANT xe2x80x9cIntermetallic Compounds in Metallurgical Siliconxe2x80x9d at the INFACON 6 conference, Proceedings of the 6th International Ferroalloys Congress, Cape Town, vol. 1, 1992, published by SAIMM, Johannesburg, pages 209-214, indicates the various intermetallic compounds present in the silicon and the means of controlling them. DE 4037021 by ELKEM recommends the presence of some ternary or quaternary phases containing Si, Fe, Al and Ca.
It is also known that the structure of intermetallic compounds located at silicon grain joints can be modified to improve the reactivity and selectivity of the Rochow reaction. These types of structures were proposed in Wacker chemie""s EP 0610807, and the applicant""s EP 0673880.
However, regardless of the care taken in forming, analyzing and structuring these intermetallic compounds, they are always located at the surface of the silicon grains, such that they are only effective at the beginning of the reaction.
Therefore, the applicant looked for a means of improving the reactivity and selectivity of the reaction by acting on the silicon grains themselves. This can be done by checking their phosphorus content, as described in WO 95/01303 deposited by BAYER and the applicant. However, the silicones industry continues to require silicon capable of further increasing the selectivity and reactivity of the Rochow reaction.
The object of the invention is a metallurgical silicon intended to be used for the synthesis of alkyl and aryl halogenosilanes, the structure of which is composed of primary silicon crystals and intermetallic compounds based essentially on silicon, iron, aluminum and calcium, and is characterized in that more than 90% of the primary silicon grains have an aluminum content of between 50 and 1000 ppm. This structure is preferably obtained with a silicon with a global aluminum content by weight of between 0.12 and 0.30%, and with a silicon solidification process after casting capable of dropping below 1200xc2x0 C. in less than 10 seconds.
In particular, since silicon is used as a semiconductor in electronic applications, it is known that the solubility of most elements in solid silicon is very low; the solubility of aluminum is of the order of 15 ppm. When metallurgical silicon obtained in the liquid state is solidified, the excess quantity of aluminum that cannot pass into solid solution in the primary silicon collects at grain joints in the form of secondary intermetallic compounds with a high aluminum content.
The applicant has found that for a given range of aluminum contents and under particular conditions for solidification of the liquid silicon, it is possible to increase the aluminum content of the primary silicon crystals beyond the normal saturation limit of 15 ppm, and to control the super-saturation level by adjusting the content of aluminum in the liquid silicon and its solidification rate, in order to increase the reactivity of silicon in the Rochow reaction.
One way of measuring the aluminum content in solid solution in primary silicon is to use an ionic probe (SIMS=Secondary Ion Mass Spectrometry). According to one normal method, when SIMS is used for a quantitative analysis, a sensitivity factor RSF is determined for aluminum such that the concentration C (in atoms per cm3) is equal to the product RSFxc3x97IAl/ISi, in other words the product of the sensitivity factor by the ratio of measured intensities for aluminum and the silicon matrix.
This RSF factor is obtained by taking the average of at least five measurements made on pre-implanted standards with a known concentration, and is of the order of 3.3 1023.
The super-saturation level of aluminum in primary silicon increases with the aluminum content in the initial liquid silicon and with the solidification rate. Thus, if 4 mm thick silicon is cast on a water cooled copper plate, resulting in complete solidification in less than 10 seconds, the following values are obtained (by weight):
By reducing the cast thickness to 2 mm, which results in complete solidification in less than 2.5 s, the following values are obtained:
Experience also shows that the results obtained depend mainly on the cooling rate between the liquid state (about 1415xc2x0 C.) and 1200xc2x0 C., the cooling rate between 1200xc2x0 C. and the ambient temperature not having very much influence. These operating conditions are completely different from the conditions mentioned in BAYER""s EP 0617039, which recommends fast cooling between 700xc2x0 C. and 120xc2x0 C. Thus, of removing solid silicon from the mold when the silicon is still red after casting on a water cooled copper plate does not significantly modify the level of super-saturation in aluminum in silicon crystals, compared with removal of the silicon from the mold at ambient temperature.
Furthermore, the cooling rate, particularly between 1400 and 1200xc2x0 C., also affects the percentage of primary silicon crystals with an aluminum content exceeding 50 ppm, very high rates giving more than 95% of super-saturated crystals, or even percentages close to 100%.
Examination with an electron scanning microscope and by X-ray diffraction of silicon crystals super-saturated in aluminum shows the existence of deformations in the crystals, such as dislocations or shear planes. For a constant content of intermetallic compounds and with an identical content of phosphorus in the primary silicon, it is observed that the reactivity of silicon super-saturated in aluminum is higher than that obtained with a silicon for which the crystals contain the normal content of about 15 ppm in solid solution. The improvement in reactivity is not very significant below a content of 50 ppm of aluminum in the crystal. It is difficult to exceed a super-saturation rate of 1000 ppm, since the aluminum then crystallizes separately.
Under industrial solidification conditions capable of changing from the liquid state to below 1200xc2x0 C. in less than 10 seconds, these limits correspond to a total content of aluminum in the liquid silicon between 0.12 and 0.30% by weight.